[pENHALLow] NOTES ON TERTIARY PLANTS 57 



in some species for which they constitute distinctive features, or their 

 imperfect development and sporadic occurrence in others. In Pseu- 

 dotsuga this tendency finds its most complete expression, and the genus 

 may be differentiated from all those to which it is otherwise related, 

 by the constancy with which the spirals are developed, and the per- 

 fection of their formation. As to the precise phylogenetic significance 

 of this fact, we are not in a position with respect to present data, to 

 draw final conclusions, but two explanations offer possible lines along 

 which solution may be reached. In the one case the spirals may be 

 regarded as atavistic. While this hypothesis could be supported with- 

 out difficulty from the standpoint of analogy, it does not seem to be 

 wholly justified by the extent, of development and the constancy and 

 completeness with which the spirals occur. On the other hand^ they 

 may be viewed as restigial structures representing features which have 

 been left over in the gradual development of the genus, and isolated 

 by obliteration of intermediate types which originally connected Pseu- 

 dotsuga with the Taxaceae or their prototypes. 



In Larix americana the outer tracheids of the summer wood some- 

 times develop very distinct spirals, and the same is also true of some 

 of the hard pines, notably Pinus ta?da — but in none of these cases 

 do the spirals become so constant in occurrence or so perfectly developed 

 as to constitute a reliable differential character. They are clearly 

 sporadic. Here, then, we find the tendency toward the recurrence of 

 primitive characters expressed in the most highly developed Coniferse 

 where they would seem to be expressions of the law of atavism rather 

 than of the nature of vestigial structures. 



Uniseriate Rays. — TJniseriate rays are a structural feature common 

 to all the TaxaceaB and Coniferse without exception, and their occur- 

 rence offers no evidence of special phylogenetic value. In most cases 

 the uniseriate form is maintained without variation, while in some 

 cases, as in Sequoia and Cupressus, there may be a more or less pro- 

 nounced tendency toward the formation of a more complex structure 

 as expressed in the fact that they become 3-seriate in part, in which 

 case the central portion broadens out while the extremities retain 

 their uniseriate character. This tendency is not expressed with any 

 degree of constancy, and it may appear in certain species of a genus, 

 the others of which show no tendency. It is, therefore, not possible 

 to employ it as a differential character having phylogenetic value, with 

 any degree of success. It nevertheless may be held to possess a certain 

 element of value as the first expression of a tendency toward the forma- 

 tion of those more highly organized rays of the fusiform type which are 

 distinguished by the presence of resm passages. 



